1. Field of the Invention
The present invention generally relates to nuclear fuel rod loaders for pulling fuel rods into a fuel rod skeleton and more particularly nuclear fuel rod loaders which avoid fuel rod bowing and maintain the fuel rod in tension.
2. Description of the Prior Art
In a nuclear reactor, the core includes a series of fuel assemblies each having top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a series of transverse support grids axially spaced along and attached to the guide thimbles. Each fuel assembly also has a plurality of elongated fuel rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods contain fission material and are grouped together in an array organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission. The reactor also has control rods, which are inserted into the guide thimbles to control the fission reaction. The fission reaction releases energy in the form of heat. A liquid coolant is pumped up through the core in order to extract the heat generated in the core for the production of steam, which runs electric generators.
In the construction of a fuel assembly, the guide thimbles are affixed to the grids to form a fuel assembly skeleton. The fuel rods are then loaded into the skeleton, passing through the cells of the grids. Typically, a fuel rod loader is used which pulls the fuel rods into the grids. The loader has a rod gripper, which is inserted through the skeleton and is coupled to the end plug of a fuel rod. It is then withdrawn, pulling the attached fuel rod into the skeleton. The top and bottom nozzles are then affixed to the skeleton to complete the fabrication of the fuel assembly.
Some known nuclear fuel rod loaders are described in issued U.S. Patents:
U.S. Pat. No. 4,297,776 teaches a loader which inserts fuel rods into a fuel rod grid by pulling the fuel rods into the grid using a leaf spring type collet that grabs the end of the fuel rod with the leaf springs when a puller rod is moved forward and releases the fuel rod by reversing this motion.
U.S. Pat. No. 4,747,997 teaches a loader which inserts fuel rods into a fuel rod grid by pulling the fuel rods into the grid using a rod gripper which is slit at the end to provide expandable fingers and which is extended to fit into the hollow end portion of the fuel rod. The end fingers are then expanded to grab the fuel rod by having an inner shaft move forward in the gripper mechanism to spread the fingers of the gripper end. The fuel rod is released by reversing the motion of the inner member to release the expanded fingers.
U.S. Pat. No. 5,317,609 teaches a method of inserting an array of fuel rods into a fuel rod skeleton by pulling the fuel rod assembly into the skeleton using a grabber mechanism that grabs the end of the fuel rods and pulls them therein.
Loaders of this design have fuel rod grippers, which have a tendency to have their closed slot end snag on a grid dimple during insertion of the puller into the fuel assembly skeleton. Bent and broken-away dimples have occurred. Thus a loader having a smooth collet design was needed for inserting the collet through the fuel rod skeleton as well as for pulling the fuel rods smoothly into the fuel assembly skeleton without damaging the skeleton.